The present invention relates to a supercritical drying method and supercritical drying apparatus for suppressing collapse of a fine pattern caused by the surface tension of a rinse solution in drying after rinse processing using water.
Recently, as MOS LSIs become larger, chips become larger, and patterns in the manufacture of LSIs become finer. At present, patterns having line widths smaller than 100 nm can be formed. The decrease in line width enables forming a pattern having a high aspect ratio (height/width). Fabricating large-scale, high-performance devices such as LSIs requires hyperfine patterns.
This hyperfine pattern includes, e.g., a resist pattern photosensitive to light, X-rays, or an electron beam that is formed through exposure, developing, and rinse processing. Further, the hyperfine pattern includes an etching pattern made of an inorganic material such as an oxide that is formed through etching, water rinsing, and rinse processing by selective etching using a resist pattern as a mask. The resist pattern can be formed by processing a photosensitive resist film of an organic material by lithography. When the photosensitive resist film is exposed, the molecular weight or molecular structure in the exposed region changes to generate a difference in solubility in a developing solution between the exposed region and an unexposed region. Developing processing using this difference can form a pattern finer than the photosensitive resist film.
If this developing processing continues development, the unexposed region also starts dissolving in the developing solution, and the pattern disappears. To prevent this, rinse processing using a rinse solution is performed to stop developing. Finally, the pattern is dried to remove the rinse solution, thereby forming the resist pattern as a processed mask on the resist film.
A serious problem in drying in forming a fine pattern is pattern collapse as shown in FIGS. 11A to 11C.
A fine resist pattern having a high aspect ratio is formed through rinse cleaning and drying after development. A fine pattern having a high aspect ratio is also formed from a material other than the resist. For example, when a substrate pattern having a high aspect ratio is to be formed by etching a substrate using a resist pattern as a mask, the substrate is cleaned after etching, and a substrate pattern 1102 is dipped in and rinsed with water 1103 together with a substrate 1101, as shown in FIG. 11A. After that, the substrate pattern 1102 is dried.
However, as shown in FIG. 11B, a bending force (capillary force) 1105 acts in drying due to the pressure difference between the water 1103 left in the substrate pattern 1102 and external air 1104. As a result, as shown in FIG. 11C, pattern collapse of the substrate pattern 1102 occurs on the substrate 1101. The collapse phenomenon becomes prominent for a pattern having a higher aspect ratio. The capillary force is reported to depend on the surface tension of the rinse solution, such as water, remaining between patterns (reference: Applied Physics Letters, Vol. 66, pp. 2655-2657, 1995).
This capillary force not only collapses a resist pattern made of an organic material, but also distorts a stronger pattern made of an inorganic material such as silicon. Therefore, the problem of capillary force caused by the surface tension of the rinse solution is serious. The problem due to the capillary force can be solved by processing using a rinse solution whose surface tension is low. For example, when water is used as a rinse solution, the surface tension of water is about 72xc3x9710xe2x88x923 N/m. On the other hand, the surface tension of methanol is about 23xc3x9710xe2x88x923 N/m. Pattern collapse is, therefore, suppressed when ethanol is dried after water is substituted by ethanol, compared to direct water drying.
Pattern collapse is more effectively suppressed when perfluorocarbon having a surface tension of 20xc3x9710xe2x88x923 N/m is used, a rinse solution is substituted by a perfluorocarbon solution, and then perfluorocarbon is dried. Although generation of pattern collapse can be reduced using a rinse solution having a low surface tension, pattern collapse cannot be eliminated using a liquid because the liquid has surface tension to a certain extent. To eliminate pattern collapse, a rinse solution whose surface tension is 0 is used, or a rinse solution is substituted by a liquid whose surface tension is 0 and then the substituted liquid is dried.
An example of the liquid whose surface tension is 0 is a supercritical fluid. The supercritical fluid is a substance at a temperature and pressure exceeding the critical temperature and critical pressure. The supercritical fluid has a dissolving power almost equal to that of a liquid, but exhibits a tension and viscosity almost equal to those of a gas. The supercritical fluid can be said to be a liquid which maintains a gaseous state. Since the supercritical fluid does not form any gas-liquid interface, its surface tension is 0. Hence, drying in the supercritical state is free from any concept of surface tension, and pattern collapse does not occur.
The supercritical fluid has both diffusion of a gas and solubility (high density) of a liquid, and can change in state from a liquid to a gas without the mediacy of any equilibrium line. If the supercritical fluid is gradually discharged in a state in which the supercritical fluid is filled, no liquid-gas interface is formed, and a pattern can be dried without any surface tension acting on a hyperfine pattern to be dried.
The supercritical fluid has been used for 10 years originally as an impurity extraction means. For example, the supercritical fluid is used as a caffeine extraction medium in a plant where caffeine is extracted from coffee. Since the solubility of the supercritical fluid changes by a set pressure for obtaining the supercritical state, the pressure can be changed to easily adjust the solubility to a substance to be extracted. For this reason, the supercritical fluid is used for extraction of caffeine. Extraction using a supercritical fluid such as carbon dioxide need not discharge any solvent waste in comparison with extraction using an organic solvent, and is evaluated as an easy-to-use extraction means. At present, supercritical extraction has been studied and put into practical use.
As a supercritical fluid, carbon dioxide which is low in critical point and safe in many cases is used. When the supercritical fluid is used for drying, a rinse solution in which a substrate surface is dipped at room temperature or less is substituted by liquid carbon dioxide in a sealed vessel, and drying starts. Since carbon dioxide liquefies at room temperature at a pressure of about 6 MPa, this substitution is done by increasing the internal pressure of the vessel to about 6 MPa. After the substrate is completely covered with liquid carbon dioxide, the interior of the vessel is set to a temperature and pressure equal to or higher than the critical point of carbon dioxide (critical point of carbon dioxide; 31xc2x0 C., 7.3 MPa), and liquid carbon dioxide is converted into supercritical carbon dioxide.
Finally, part of the vessel is opened to externally discharge supercritical carbon dioxide, the interior of the vessel is reduced to the atmospheric pressure, and supercritical carbon dioxide in the vessel is gasified to end drying. In pressure reduction, carbon dioxide does not liquefy but gasifies, so no gas-liquid interface on which surface tension acts is formed on the substrate. For this reason, hyperfine patterns on the substrate can be dried without any collapse.
An example of the supercritical drying apparatus is an apparatus constituted by connecting a cylinder 1203 storing liquid carbon dioxide via a valve 1204 to a reaction chamber 1202 in a sealable vessel 1201, as shown in FIG. 12. In this apparatus, the valve 1204 on the liquid carbon dioxide inlet side is opened to introduce liquid carbon dioxide into the reaction chamber 1202, and a valve 1205 on the discharge side is adjusted to limit the amount of liquid carbon dioxide discharged from the reaction chamber 1202, thereby controlling the internal pressure of the reaction chamber 1202.
While liquid carbon dioxide is directly introduced from the cylinder 1203 into the reaction chamber 1202, the vessel 1201 is heated to, e.g., about 31xc2x0 C., the valve 1205 is adjusted to decrease the amount of liquid carbon dioxide discharged from the reaction chamber 1202, and the pressure of the reaction chamber 1202 is set to 7.38 MPa or more. Then, liquid carbon dioxide in the reaction chamber 1202 changes to the supercritical state. After that, the valve 1204 is closed, and the valve 1205 is opened to decrease the internal pressure of the reaction chamber 1202. Supercritical carbon dioxide in the reaction chamber 1202 is gasified to end supercritical drying.
As shown in FIG. 13, a pressure pump 1206 can be arranged between the cylinder 1203 and the valve 1204 to control the internal pressure of the vessel 1201 to be higher.
Generally in a pattern formation process by photolithography, a substrate is often rinsed with water and dried at last. However, since water hardly dissolves in liquid carbon dioxide, water is substituted by ethanol which is relatively easily miscible with carbon dioxide, and then supercritical drying is executed. Although ethanol is miscible, the solubility of carbon dioxide and an alcohol such as ethanol is insufficient, and substitution requires a long time. Further, some resists dissolve in alcohols, so no alcohol can be used after water rinsing.
It is, therefore, the principal object of the present invention to enable drying while suppressing generation of pattern collapse.
To achieve the above object, according to one aspect of the present invention, a pattern layer having a predetermined pattern formed on a substrate is exposed to water. While water attaches to the pattern layer, the pattern layer is exposed to a predetermined liquid to emulsify water attached to the pattern layer with the liquid, thereby obtaining a state in which the liquid attaches to the pattern layer. The liquid attached to the pattern layer is substituted by a liquid of a nonpolar substance which is gaseous in the atmosphere. Then, the nonpolar substance attached to the pattern is changed to the supercritical state, and the supercritical nonpolar substance attached to the pattern layer is gasified.
According to the present invention, water attached to the pattern layer formed on the substrate is substituted by the liquid. In attaching the liquid of the nonpolar substance to the pattern layer, no water attaches to the pattern layer.
According to another aspect of the present invention, a pattern layer having a predetermined pattern formed on a substrate is exposed to water. While water attaches to the pattern layer, the pattern layer is exposed to an alcohol liquid to dissolve water attached to the pattern layer in the alcohol liquid, thereby obtaining a state in which the alcohol liquid attaches to the pattern layer. While the alcohol liquid attaches to the pattern layer, the pattern layer is exposed to an aliphatic hydrocarbon liquid to dissolve the alcohol liquid attached to the pattern layer in the aliphatic hydrocarbon liquid, thereby attaining a state in which the aliphatic hydrocarbon liquid attaches to the pattern layer. While the aliphatic hydrocarbon liquid attaches to the pattern layer, the pattern layer is exposed to a liquid of a nonpolar substance which is gaseous in the atmosphere. The aliphatic hydrocarbon liquid attached to the pattern layer is dissolved in the liquid of the nonpolar substance to set a state in which the liquid of the nonpolar substance attaches to the pattern layer. Then, the nonpolar substance attached to the pattern is changed to the supercritical state, and the supercritical nonpolar substance attached to the pattern layer is gasified.
According to the present invention, alcohol attached to the pattern layer formed on the substrate is substituted by an aliphatic hydrocarbon. In attaching the liquid of the nonpolar substance to the pattern layer, the aliphatic hydrocarbon attaches to the pattern layer.
According to still another aspect of the present invention, a pattern layer having a predetermined pattern formed on a substrate is expose to water. While water attaches to the pattern layer, the pattern layer is exposed to a nonpolar aliphatic hydrocarbon liquid to emulsify water-attached to the pattern layer with the aliphatic hydrocarbon liquid, thereby obtaining a state in which the aliphatic hydrocarbon liquid attaches to the pattern layer. While the aliphatic hydrocarbon liquid attaches to the pattern layer, the pattern layer is exposed to a liquid of a nonpolar substance which is gaseous in the atmosphere. The aliphatic hydrocarbon liquid attached to the pattern layer is dissolved in the liquid of the nonpolar substance to attain a state in which the liquid of the nonpolar substance attaches to the pattern layer. After that, the nonpolar substance attached to the pattern is changed to the supercritical state, and the supercritical nonpolar substance attached to the pattern layer is gasified.
According to the present invention, water attached to the pattern layer formed on the substrate is substituted by an aliphatic hydrocarbon. In attaching the liquid of the nonpolar substance to the pattern layer, the aliphatic hydrocarbon attaches to the pattern layer.
According to still another aspect of the present invention, as the first step, a pattern layer having a predetermined pattern formed on a substrate is exposed to water. As the second step, while water attaches to the pattern layer, the pattern layer is exposed to a liquid of a nonpolar substance which is gaseous in the atmosphere, and water attached to the pattern layer is emulsified with the liquid of the nonpolar substance. As the third step, the nonpolar substance attached to the pattern layer is changed to the supercritical state. Finally as the fourth step, the supercritical nonpolar substance attached to the pattern layer is gasified.
According to the present invention, water attached to the pattern layer is substituted by the liquid of the surfactant-added nonpolar substance, and the nonpolar substance gasifies on the surface of the pattern layer after changing to the supercritical state. In drying the pattern layer, no surface tension is generated on the gas-liquid interface.
A supercritical drying apparatus according to the present invention comprises a sealable vessel having a reaction chamber for placing a substrate to be processed, nonpolar substance supply means for supplying a liquid of a nonpolar substance which is gaseous in the atmosphere to the reaction chamber, surfactant addition means for adding a surfactant to the nonpolar substance supplied to the reaction chamber, discharge means for discharging a fluid introduced into the reaction chamber, control means for controlling the internal pressure of the reaction chamber until the nonpolar substance changes to the supercritical state, and temperature control means for controlling the internal temperature of the reaction chamber to a predetermined temperature.
According to the present invention, the liquid of the surfactant-added nonpolar substance is supplied to the reaction chamber, and the nonpolar substance supplied to the reaction chamber changes to the supercritical state.